Generally speaking, a circulating fluidized bed boiler comprises a furnace for combusting fuel, an outlet channel connected to the upper section of the furnace for the discharge of flue gas out of the furnace, a solids separator for receiving the flue gas via the outlet channel from the furnace, and for separating solid particles from the flue gas. The CFB boiler further comprises at the lower portion of the solids separator, a return channel for taking the hot solids separated by means of the solids separator to the lower section of the furnace, and at the upper portion of the solids separator, a flue gas duet for removing cleaned flue gas to the backpass of the boiler, to gas cleaning devices and, further, through the stack to the environment.
The outlet channel, solids separator, and the return channel form a so-called external hot circulation, in which the hot solids entrained in the flue gas are first taken out of the furnace, then treated in the separator, and finally, returned to the furnace. Most often, somewhere in the external circulation, in flow communication with the solids return channel, a fluidized bed heat exchanger is arranged. The heat exchanger may be supported to the lower portion of the solids separator such that the return channel takes the solids from the heat exchanger to the lower section of the furnace. Or, the heat exchanger may be supported by a side wall of the furnace, such that the return channel takes the solids from the solids separator to the heat exchange chamber. As to the fluidized bed heat exchangers, they may also be arranged in the internal circulation, i.e., for receiving the solids from the bed material flowing down along the furnace walls. And, naturally, there are also fluidized bed heat exchangers that may receive solids from either the internal or the external circulation, or simultaneously, from both circulations.
The lower section of the furnace is provided with feeds for feeding fuel, inert bed material, and possibly sulfur binder to the furnace, and, finally, the bottom of the furnace is provided with feeds for feeding oxide-containing fluidizing gas into the furnace, in other words, a gas inlet channel, a wind box, and nozzles.
Published PCT International Application No. WO 2007/128383 A2 discusses a fluidized bed heat exchanger structure for a CFB boiler. The CFB boiler of the PCT publication, or in fact, the fluidized bed heat exchanger, comprises two heat exchange chambers arranged in series in communication with the return channel, such that a first fluidized bed heat exchange chamber supported below the solids separator receives hot solids directly, actually, via a gas seal, from the solids separator, and then, in normal conditions, discharges the cooled solids to a second fluidized bed heat exchange chamber arranged in connection with the wall of the lower section of the furnace. Finally, the cooled solids are returned to the furnace from the second heat exchange chamber.
In accordance with the teachings of the PCT publication, the upper heat exchange chamber is also provided with a return for returning cooled solids from the upper heat exchange chamber directly to the furnace. Both heat exchange chambers have internal heat exchange surfaces arranged within the heat exchange chambers for cooling the solids before they are returned to the lower section of the furnace. In other words, the two heat exchange chambers discussed above are connected in series in the external solids circulation of a CFB boiler.
It is a specific feature of the second, i.e., the lower heat exchange chamber, of the above-mentioned PCT publication that the heat exchange chambers may receive hot solids, not only from the first heat exchange chamber, but also, from the internal circulation, i.e., the second heat exchange chamber is provided with an inlet arranged in the wall of the lower section of the furnace such that hot solids flowing down along the boiler walls are able to enter the second fluidized bed heat exchange chamber. Further, the heat exchanger arrangement of the PCT publication is provided with an overflow for allowing overflow of solids from the first heat exchange chamber directly to the second heat exchange chamber, in a case that the solids flowing into the first heat exchange chamber is larger than the discharge flowing out of the first heat exchange chamber. In connection with this discussion concerning the heat exchangers, it should be understood that a large CFB boiler is usually provided with several parallel solids separators and heat exchangers connected to their return channel either on one side of the boiler or on both sides thereof, but, for clarity reasons, both above and in the following description of the invention, mainly, only one heat exchange arrangement with one solids separator, is discussed.
The starting point in the development of the fluidized bed heat exchanger of the PCT publication discussed above was to be able to construct a heat exchange arrangement that may be used in almost all possible applications due to its versatile controlling possibilities. A problem the construction of the PCT publication solved related to the traditional location of the fluidized bed heat exchange chambers on the outside walls of the lower section of the furnace. While the CFB boilers grew, it was not possible to increase the size of the fluidized bed heat exchange chambers accordingly, as increasing the height of a heat exchange chamber resulted in the increase of pressure losses in the fluidization air, and an increase in the width of the heat exchanger was not possible due to a lack of space. Thus, the growing size of the CFB boilers was taken into account in the PCT publication by arranging the heat exchangers one on top of the other, whereby requirements relating to both the available space and the acceptable pressure Josses were taken into account. And, finally, the adjustability or controllability of the heat exchange arrangement was ensured by providing the arrangement with equipment giving a possibility to run the arrangement in several different ways.
When all the above-discussed and other considerations were taken into account in the design of the heat exchange arrangement, however, the construction of the arrangement became less optimal for some specific applications. Such applications are cases when no extensive control is required, or cases when the connection of the heat exchange chambers in series is not desired, for some reason. In other words, the prior art arrangement has a few drawbacks or problems.
First, since the upper heat exchange chamber is supposed to discharge the cooled solids to the lower one, the channel between the heat exchange chambers runs between the upper heat exchange chamber and the furnace forcing positioning of the first/upper heat exchange chamber substantially far from the furnace wall. This also means that the solids separator has to be positioned far from the furnace, as the upper heat exchange chamber is normally positioned right below the separator, and supported from the separator.
Second, as the lower heat exchange chamber is supposed to be able to receive all the cooled solids from the upper heat exchange chamber, and possibly, also some additional solids from the internal circulation, it is clear that the volume of the lower heat exchange chamber should at least correspond to the one of the upper heat exchange chamber. As already discussed above in connection with the PCT publication (WO 2007/128883 A2), neither the height nor the width (in a direction parallel to the furnace wall) of the lower heat exchanger can be chosen freely, but both the pressure loss in the fluidization, and the space occupied by the heat exchange chamber have to be considered. The above consideration results in that the dimensions of the lower heat exchange chamber are substantially equal with the upper one. Thereby, there is very little room in connection with the lower section of the furnace for the equipment necessary for running the boiler, such as, for example, the start-up burner, a temperature measuring device for measuring the lower furnace temperature, a pressure measurement device for measuring the bed pressure, and feeds for introducing fuel, bed material, secondary air, additives, recirculated flue gas (if in use), etc.
Third, due to the various running alternatives, i.e., control options in the prior art boiler, there are conduits and channels for each alternative. For instance, the upper heat exchange chamber has one inlet from the separator, and several outlet channels and lift channels. One lift channel and outlet channel leading to the lower heat exchanger, another lift channel and outlet channel leading to the furnace, and overflow channels leading to both the lower heat exchange chamber and to the furnace. In addition to the channels, rather complicated fluidization means and controllers for adjusting the fluidizations are also required at the bottom of the upper heat exchange chamber. If, and when, the various channels and conduits require bellows to separate components in different temperatures, the bellows, again, occupy space, and also increase the costs of the heat exchanger arrangement together with the already numerous channels, conduits, fluidization equipment, and control systems that have been discussed above. And, still further, all of the channels and conduits need to be either made of water/steam tube walls and connected to the rest of the steam/water system, or made of a refractory material. Irrespective of the manufacture, this adds to the expenses as constructing the channels of water/steam tube walls or refractory material is a complicated and time-consuming task.
For the above reasons, it has been found necessary to improve the construction of a CFB boiler and its heat exchanger arrangement.